High frequency power source

ABSTRACT

A high frequency electrosurgical power generator configured to produce electrical power at a frequency of about 1 to about 14 MHz and preferably having an essentially sinusoidal waveform with a voltage level up to 1,000 Vrms, and a current level up to 5 Amps. The output of the high frequency electrosurgical power generator is connected to an electrosurgical tool configured to receive the voltage and current produced by the electrosurgical power generator and deliver the voltage and current to an electrosurgical site. The output of the electrosurgical generator preferably is an essentially sinusoid waveform with a frequency between about 3 MHz and about 8 MHz, up to about 700 volts rms, up to about 2 amps, with a total power of up to 1,000 watts.

RELATED APPLICATIONS

This application is a divisional application of prior application Ser.No. 10/658,572, filed Sep. 9, 2003, now U.S. Pat. No. 7,175,618, whichis divisional of application Ser. No. 09/752,978, filed Dec. 28, 2000,now U.S. Pat. No. 6,620,157 from which all priority is claimed and whichare incorporated herein by reference in their entirety.

BACKGROUND OF THE INVENTION

Power generators used in electrosurgical procedures deliver electricalenergy to an electrosurgical tool for operating on the tissue of apatient. An active electrode of the tool, connected to the powergenerator, concentrates the delivery of the electrical energy to arelatively small region of tissue of the patient. The electrical energytypically includes energy in the radio frequency (RF) band. Theconcentration of electrical energy facilitates cutting or coagulation ofthe tissue of the patient. During typical operation of a monopolarelectrosurgical device, an alternating electrical current from thegenerator flows from an active electrode to a return electrode bypassing through the tissue and bodily fluids of a patient.

During an electrosurgical operation, different tissue types may beencountered, such as, for example, fat, connective, glandular andvascular tissues. Connective, glandular and vascular tissues can havesimilar characteristics in the way they react to electrical energy,specifically, they have similar characteristics of electrical impedance.Fat however, has significantly different electrical responsecharacteristics. In particular, fat presents a higher impedance to theflow of electrical current than do the other types of tissues. Thetissue of certain anatomical portions, or regions, of a patient's bodymay be largely heterogeneous on a macroscopic scale, such as on a scalecommensurate with that of an electrosurgical cutting tool. For example,breast tissue has this heterogeneous property and can be made up of allthe tissue types discussed above, i.e., fatty, glandular, connective andvascular tissues. The variations in electrical impedance exhibited bythese various tissue types can be problematic when attempting to performelectrosurgical cutting in such heterogeneous, or non-homogeneous,tissue.

In a typical electrosurgical procedure, the amount of electrical energydelivered by a power generator must be carefully controlled. Ifinsufficient power is delivered by the power generator, the tissuecutting of the electrosurgical procedure will be inhibited. If morepower than necessary is delivered by the power generator there mayexcessive, and unnecessary, collateral tissue damage making it moredifficult to perform a histology on a sample and thereby decreasing theability of a pathologist to diagnosis the sample, as well as resultingin a more difficult recovery by the patient in addition to othersequela. Using a regulated power generator helps control and stabilizethe electrical energy delivered into the patient's tissue. However, dueto the different electrical response characteristics of the varioustissue types that may be present, the energy coupled into the tissue mayvary even if the power generator is regulated. Generally, typical RFpower generators experience difficulty in cutting through fattynon-homogeneous tissue because of the non-homogeneous tissue types thatare typically encountered.

In addition, typical RF power generators are only effective with toolshaving small cutting surfaces. Thus, during an electrosurgicalprocedure, if fat is encountered, a surgeon must perform surgical cutsby “feathering”, making repetitive shallow cuts with countertractionover the same area to attain a desired depth of cut. In addition,because typical power generators are only effective for tools with smallcutting surfaces, the types of tools available to a surgeon duringelectrosurgery are limited.

There is a need in the art for improved electrosurgical RF powergenerators that can be used with electrosurgical tools that encounternon-homogeneous tissue, such as, for example, breast biopsy instruments.Electrosurgical tools, such as electrosurgical breast biopsyinstruments, can present varying load requirements to an electrosurgicalpower generator than typical electrosurgical tools, due to theheterogeneous nature of the tissue they are used to cut or coagulate.

From the discussion above, it should be apparent that there is a needfor an electrosurgical power generator used in electrosurgicalprocedures that will more effectively couple electrical energy todifferent types of tissue, in particular heterogeneous tissue thatincludes fat tissue. In addition, there is a need for a power generatorthat works effectively with large cutting surfaces, thereby expandingthe types of tools that are available for electrosurgery.

The present invention satisfies these and other needs.

SUMMARY OF THE INVENTION

The invention is directed to a high frequency electrical power generatorparticularly suitable for use in electrosurgery.

An electrical power generator constructed in accordance with theinvention is configured to produce electrical power at a frequency ofabout 1 MHz to about 14 MHz, preferably about 3 MHz to about 8 MHz. Theelectrical power generated preferably has an essentially sinusoidalwaveform with a total harmonic distortion (THD) of less than 5%. Thespecified frequency and waveform help to minimize damage to adjacenttissue during electrosurgery. The power output has a high voltage level,for example, up to about 1,000 Vrms, and a high current level forexample, between about 0.5 amps to 5 amps, particularly about 1 amp to 2amps. The output of the electrosurgical power generator is connected toan electrosurgical tool configured to receive the voltage and currentproduced by the electrosurgical power generator and deliver the voltageand current to an electrosurgical site on a patient. There is preferablyat least one ground pad in electrical contact with the patient tocomplete an electrical circuit for the system comprising the generatorand the tool, thereby providing a controlled return path for the currentfrom the electrosurgical site to the electrosurgical power generator.

The system may also include a distal interface pod, located proximate tothe electrosurgical site, connected to the output of the high frequencyelectrosurgical generator. In one embodiment, the distal interface podis configured to present a desired load to the electrosurgical powergenerator. In addition, the distal interface pod may include safety andpatient interface functions, as well as telemetry functions such asmonitoring various parameters important to safety as well as controlparameters, for example, the voltage and current produced by theelectrosurgical power generator and delivered to the electrosurgicaltool.

The high frequency characteristics described above improve theelectrosurgical power generator's ability to deliver consistent powerover a range of electrical impedance loads caused by variations intissue types.

The high voltage characteristics described above facilitate the use oflong electrodes during an electrosurgical procedure. Use of longelectrodes may require substantially higher starting and sustainingvoltages, particularly when fatty tissue is present, in contrast to whenother types of tissue are encountered.

Electrical power output with an essentially sinusoidal waveform in anelectrosurgical procedure concentrates the electrical power into cuttingtissue, thereby reducing the total power required during theelectrosurgical procedure. Reduction in total power results in lessheating, and thereby less damage to collateral tissue. Total powerdelivered during an electrosurgical procedure may also be reducedthrough duty factoring where the power is turned “on” and then “off” inrapid succession. When duty factoring the power output, the waveshapeenvelope may be, for example, a ramped, or trapezoidal rectangle, or azero crossing switched rectangle.

These and other features of the invention will become more apparent fromthe following detailed description of the invention and the accompanyingexemplary drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating a power delivery systemconstructed in accordance with the invention for use in anelectrosurgical procedure.

FIG. 2 is a block diagram illustrating another embodiment of a powerdelivery system constructed in accordance with the invention for use inan electrosurgical procedure.

FIG. 3 is a block diagram illustrating yet another embodiment of a powerdelivery system constructed in accordance with the invention for use inan electrosurgical procedure.

FIG. 4 is a block diagram of one embodiment of a control system for usein a power delivery system constructed in accordance with the invention.

FIG. 5 is a block diagram of another embodiment of a control system foruse in a power delivery system constructed in accordance with theinvention.

FIG. 6 is a block diagram of another embodiment of a control system foruse in a power delivery system constructed in accordance with theinvention.

FIG. 7 is a block diagram of yet another embodiment of a control systemfor use in a power delivery system constructed in accordance with theinvention.

FIG. 8 is a representation illustrating the power signal path during anelectrosurgical procedure.

FIG. 9 is a block diagram of one embodiment of an electrosurgical powergenerator.

FIG. 10 is a block diagram that shows additional details of oneembodiment of a control unit.

FIG. 11 is a block diagram showing additional detail of portions of usercontrols and gating and duty factor control.

FIG. 12 is a block diagram showing additional detail of portions ofsignal conditioning and error amplifier.

FIG. 13 is a block diagram of an embodiment of a distal interface pod.

FIG. 14 is an illustration an embodiment of a flexible shielded cable.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a block diagram illustrating a power delivery system 100constructed in accordance with the invention for use in anelectrosurgical procedure. The power delivery system 100 includes acontrol system 102. The control system generates a desired waveform foruse in the electrosurgical procedure. The control system 102 outputs thedesired waveform to a power delivery unit 104. The control system 102condition the waveform, including, for example, amplitude control,gating and duty factor control. Gating and duty factor control areexplained in more detail below. The power delivery unit 104 receives thedesired waveform, amplifies and provides electrical capability, andoutputs a power waveform. In one embodiment the power delivery unit maybe a voltage controlled electrical power generating unit configured toproduce a high frequency current having an essentially sinusoidalwaveform having a total harmonic distortion (THD) of less than 5%.

The power waveform output by the power delivery unit 104 is routed to atelemetry and impedance matching unit 106 via a cable 108. The telemetryand impedance matching unit 106 receives the power waveform. Thetelemetry and impedance matching unit 106 also provides an interfacethat efficiently transfers the power waveform to the electrosurgicaltool. In addition, the telemetry and impedance matching unit 106measures parameters of the power waveform and communicates thesemeasurements back to the control system 102.

In one embodiment, the parameters measured by the telemetry andimpedance matching unit 106 include the voltage and current of the powerwaveform delivered to the electrosurgical tool. In another embodiment,there is no telemetry and impedance matching unit 106. In thisembodiment, the power waveform output by the power delivery unit 104 isconnected directly to the electrosurgical tool.

FIG. 2 is a block diagram illustrating another embodiment of a powerdelivery system constructed in accordance with the invention for use inan electrosurgical procedure. In this embodiment, the power deliverysystem 200 includes a power delivery unit 204 configured to output aconstant, high power output waveform. The high power waveform is routedto a control system 206. In this embodiment, the control system 206 isconfigured to receive the high power output waveform, condition thewaveform and output a controller power waveform to the electrosurgicaltool. The conditioning of the waveform may include, for example,amplitude control, gating, and duty factor control. Gating and dutyfactor control are explained in more detail below. In anotherembodiment, there may be an optional telemetry and impedance matchingunit, as shown in FIG. 1, located between the control system and theelectrosurgical tool. This embodiment may also include measurement ofwaveform parameters sent back to the control unit to be used to improvethe power delivered to the electrosurgical tool.

FIG. 3 is a block diagram illustrating yet another embodiment of a powerdelivery system constructed in accordance with the invention for use inan electrosurgical procedure. In this embodiment, the power deliverysystem 300 includes a power delivery unit 104 and a telemetry andimpedance matching unit 106 as described in relation to FIG. 1. The FIG.3 embodiment includes a separate signal generator 304 and controller306. The signal generator 304 includes a signal source 308 configured tooutput a continuous signal at a desired frequency. The signal source 308output is routed to a waveform control unit 310. The waveform controlunit 310 receives the signal source 308 output and conditions it toproduce a desired signal to be sent to the power delivery unit 104. Theconditioning may include, for example, gating and duty factor control.The controller 306 receives measurement data of waveform parametersdelivered to the electrosurgical tool from the telemetry and impedancematching unit 106. The controller 306 adjust the operation of the signalsource 308 and the waveform control unit 310 to deliver the desiredwaveform to the electrosurgical tool as measured by the telemetry andimpedance matching unit 106.

FIG. 4 is a block diagram of one embodiment of the control system 102for use in a power delivery system constructed in accordance with theinvention. In this embodiment, the control system 102 includes a firstsignal source 402 and a second signal source 404. The two signal sourcesgenerate sinusoidal signals at a desired frequency f₀. The output of thetwo signal sources are combined in a summer 408. The summer 408 combinesthe outputs of the two signal sources 402 and 404 and outputs a combinedsignal 410 to the power delivery unit 108. A controller 406 receivesmeasurements from the telemetry and impedance matching unit (not shownin FIG. 4) that relate to the power delivered to the electrosurgicaltool. In response to the measurement signals received the controller 406adjusts the phase relationship between the two signal sources 402 and406. By adjusting the phase relationship between the two signal sources402 and 404, a desired combined signal 410 is output by the summer 408to the power delivery unit (not shown in FIG. 4).

FIG. 5 is a block diagram of another embodiment of a control system foruse in a power delivery system. In this embodiment, the control system102, as discussed in FIG. 4, includes a first signal source 402, asecond signal source 404, and a controller 406. In this embodiment, thefirst signal source 402 and the second signal source 404 operate atdifferent frequencies f₁ and f₀ respectively. The output of the twosignal sources are routed to a multiplier 502, where the two signals aremultiplied together. The output of the multiplier 502 contains, amongother components, the difference between the two frequencies. The outputof the multiplier 502 is routed to a lowpass filter 504. The lowpassfilter attenuates components in the output waveform of the multiplier502 except the component of the waveform at the difference frequencybetween the two signal sources. The output of the lowpass filter 504 isa waveform with a desired frequency of f₁-f₀. The output of the lowpassfilter 504 is routed to the power delivery unit (not shown in FIG. 5).

FIG. 6 is a block diagram of another embodiment of a control system foruse in a power delivery system in accordance with the invention. In thisembodiment, there is a first signal source 602 and a second signalsource 604. The output of the first signal source 602 is routed to amodulator 606. The modulator may adjust the phase, amplitude, or thephase and the amplitude of the signal received from the first signalsource. The adjustments made by the modulator 606 are determined by acontroller 608. For example, the controller 608 may receive telemetryand impedance matching data from the telemetry and impedance matchingunit 106 and determine a desired modulation in response to the receiveddata. The adjusted signal from the modulator 608 is routed to mixer 610.The other input to the mixer 610 is the output from the second signalsource 604. For example, the mixer 610 may sum the two waveforms. Themixer 610 combines the two waveforms and outputs a combined waveform tothe power delivery unit 104.

FIG. 7 is a block diagram of yet another embodiment of control systemfor use in a power delivery system in accordance with the invention. Inthe embodiment, the control system 102 includes a memory 702 configuredto store data to be used in generating a desired waveform. The memory702 is in communication with a digital to analog converter (ADC) 704 anda controller 706. The controller 706 is also in communication with theADC 704. A desired set of data stored in memory 702 corresponding to adesired waveform is transferred from the memory 702 to the DAC 704. Forexample, the memory 702 may be configured to be a look up tablecontaining data corresponding to various waveforms. The desired set ofdata is selected by the controller 706, for example, in response to datareceived from the telemetry and impedance matching unit 106. The DAC 704is configured to receive the desired data and to output an analogwaveform in response. In another embodiment there is no memory 702,rather the controller 706 calculates the desired data to be used by theDAC 704.

FIG. 8 is a representation illustrating the power signal path during anelectrosurgical procedure. An electrosurgical power generator 802produces a desired output so as to effectively transfer electricalenergy to the tissue of a patient 804 at a surgical site. In oneembodiment, the output of the electrosurgical power generator is a radiofrequency (RF) signal, such as, for example, an essentially sinusoidalwaveform at about 5 MHz, at a power level of 450 to 650 watts. Anessentially sinusoidal waveform may be, for example, a waveform withless than about 5% total harmonic distortion (THD). In anotherembodiment, the electrosurgical power generator output may be anessentially sinusoidal waveform at about 5 MHz at a power level up to1,000 watts. In yet another embodiment, the electrosurgical powergenerator output may be an essentially sinusoidal waveform at about 3.4MHz. It is contemplated that the electrosurgical power generator outputmay be any specific frequency between about 1 MHz and about 14 MHz, andvarious power levels up to several kilowatts.

In one embodiment, the output of the electrosurgical power generator 802is connected to a distal interface pod 806 via a shielded cable 808. Thedistal interface pod 806 is located in relatively close proximity to thepatient 804. As described below, the distal interface pod 806 comprisesvarious electrical components and circuits providing, such as, forexample, impedance matching and sensing circuits. In another embodiment,there is no distal interface pod, with the output of the electrosurgicalpower generator connected directly to the electrosurgical tool 810.

The output of the distal interface pod 806, or the output of theelectrosurgical power generator 802, is connected to the electrosurgicaltool 810 via a flexible shielded cable 812. As described below, theflexible shielded cable 812 conducts the output of the distal interfacepod 806 to the electrosurgical tool 810 providing shielding for thesurgeon and patient from radiated emissions produced by the RF waveform.In addition, the flexible cable 812 reduces the force required by thesurgeon in maneuvering the electrosurgical tool 810 thereby increasingthe effectiveness with which the surgeon can manipulate theelectrosurgical tool 810.

During an electrosurgical procedure a surgeon will maneuver theelectrosurgical tool 810 about the patient 804 to produce the desiredresults. The electrosurgical power generator procedures an electricalcurrent that flows from the electrosurgical tool 110, through thepatient 804 to a ground pad 814. The ground pad 814 is connected to theelectrosurgical power generator 802 via the distal interface pod 806 tocomplete an electrical circuit. In addition, a pad sense circuit, in thedistal interface pod 806, provides a mechanism for detecting that theground pad is properly positioned to the patient.

Typically, due in part to the limited area available and the number ofpeople involved, during an electrosurgical procedure the electrosurgicalpower generator 802 will be at a remote location, away from the patient,perhaps in a room separated from the operating room. A remote switch 820in the operating room may be used providing the surgeon control of whenthe electrosurgical power generator outputs power to the electrosurgicaltool 810 as well as whether to provide cut, coagulation or blend energy.In one embodiment, the remote switch 820 may be a foot activated switch.In other embodiments, other types of remote switches may be used, suchas, for example, a voice activated switch or a push button located onthe electrosurgical tool 810, or other related control equipment.

FIG. 9 is a block diagram of one embodiment of a electrosurgical powergenerator 802. A control unit 902 produces a control signal used tocontrol a desired output from the electrosurgical power generator 802.The control unit includes user controls 904 allowing a user to selectspecific, desired setting of the control signal. In one embodiment, theuser inputs desired setting via, such as, for example, a keyboard,keypad, touch screen, switches, rotary devices or any combination ofthese types of devices.

In one embodiment, the user controls 904 allow the user to select adesired frequency of the control signal output to the electrosurgicalpower generator 802. For example, the user controls may allow selectionof an output frequency of 5 MHz or 3.4 MHz. In another embodiment, otherparameters of the control signal may be selected by the user, such as,for example, output power level, output voltage level, output currentlevel, duty cycle and gating controls of the output signal. Usercontrols 904 may also receive input from a remote switch 820 allowingremote operation of the electrosurgical power generator 802. Adjustmentof the above parameters allows for different modes of operation such ascut, coagulation and blend.

The output of the user controls is in communication with the signalgenerator 906 and the feedback circuit 908. The signal generator 906produces a representation of the desired output of the electrosurgicalpower generator 802. The signal generator 906 is configured to acceptinput from the user control 904, and the feedback circuit 908, andmodify its output accordingly. The signal generator 908 output is a lowpower signal used to control at least one power amplifier.

The feedback circuit 908 is configured to accept signals from the usercontrols 904 and the distal interface pod 806. Signals from the distalinterface pod 806 may include signals for sensing various parameters ofthe RF waveform at the distal interface pod 806, such as, for examplecurrent and voltage present at the electrosurgical tool 810. Thefeedback circuit 908 outputs a signal to the signal generator 906 suchthat signal generator output controls at least one power amplifier toproduce a desired waveform to the electrosurgical tool 810, as indicatedby sensing parameters of the RF waveform at the distal interface pod806.

The output of the control unit is in communication with the powerdelivery unit 909. The power delivery unit 909 is configured to receivethe output of the control unit and produce a desired RF power outputthat is transmitted to an electrosurgical tool for use in anelectrosurgical procedure.

In one embodiment, the power delivery unit 909 includes a single poweramplifier module 912. In other embodiments, different numbers of poweramplifier modules 912 may be included in the power delivery unit 909.For example, power delivery unit may include two, four, eight, oranother number of power amplifier modules 912. In embodiments thatinclude more than one power amplifier modules 912, the power deliveryunit 909 may also include a splitter 910, and a power combiner 916.

The power delivery unit 909 illustrated in FIG. 9 includes four poweramplifier modules 912. In this embodiment, the output of the controlunit 902 is in communication with a splitter 910. The splitter 910receives the low power output signal from the control unit 902, buffersthe signal, and outputs a plurality of duplicate signals, one for eachpower amplifier module 912, of the same magnitude and characteristics asthe signal from the control unit. In this embodiment the splitter 910outputs four duplicate signals. Each of the duplicate outputs of thesplitter 910 is connected to an individual power amplifier module 912.The power amplifier modules 912 are configured to receive a low powersignal, and amplify the signal to a desired power level.

In one embodiment the power amplifier module 912 is an RF poweramplifier, such as, for example, an LCF Enterprises part number30-1-150-35-ES, or an equivalent RF power amplifier, adapted to producethe desired frequency, for example, about 1 MHz to about 10 MHz,specifically about 3 MHz to about 8 MHz, and more specifically about 3.4MHz to about 5 MHz.

In one embodiment, the power amplifier 912 is an AB linear amplifier. Inanother embodiment, the power amplifier 912 is a class AB amplifier. Inother embodiments the power amplifier 212 may be a class E, class B,class C or class D amplifier. In another embodiment, the power amplifier912 outputs an essentially sinusoidal waveform with less than about 5%total harmonic distortion (THD).

In one embodiment, each of the high power output of each power amplifiermodule 912 is connected to a filter 914. Each filter 914 is configuredto accept the output of the power amplifier module 912 and eliminateundesired spectral components. For example, the filter 914 may be a lowpass filter with a corner frequency of about MHz, a rolloff of about 24dB per octave. In another embodiment, various parameters of the filterare selected so that the output waveform is such that subsequentmatching and telemetry isolation transformers perform adequately. Inanother embodiment, the filter 914 is a bandpass filter centered atabout the fundamental frequency of signal generator 906.

Each of the filters 914 outputs are connected to a power combiner 916.The power combiner 916 is configured to accept the outputs from theplurality of power amplifiers 912. In one embodiment, the power combiner916 is configured to accept four independent power signals. Theindependent power signals are summed in the power combiner 916 into onepower signal. The output of the power combiner 916 is transmitted to thedistal interface pod 806 via the shielded cable 808. In an embodimentwhere a single power amplifier is used, then a power combiner is notnecessary.

FIG. 10 is a block diagram that shows additional details of oneembodiment of a control unit 902. The control unit 902 includes anoscillator 1002. The oscillator 1002 generates a waveform at a desiredfrequency. Generally, the oscillator is configured to output a periodicwaveform. In one embodiment, the oscillator 1002 produces squarewaveform of about 5 MHz. In another embodiment, the oscillator 1002produces sinusoidal waveform of about 5 MHz. In other embodiments, theoscillator 1002 produces square waveforms or sinusoidal waveforms atdifferent frequencies from about 1 MHz to about 14 MHz, specificallyabout 3 MHz to about 8 MHz, and more specifically about 3.4 MHz to about5 MHz. In yet another embodiment, the oscillator may be the scaledoutput of a higher frequency signal, for example, a 40 MHz clock dividedby 8 to produce a 5 MHz squarewave. The oscillator output may beconnected to a filter 1004.

The filter 1004 is configured to receive the output of the oscillator1002. In one embodiment, the filter 1004 is configured as a low passfilter with a corner frequency of about 7 MHz, a rolloff ofapproximately 12 dB per octave. In another embodiment, the filter 1004is a bandpass filter centered at about the fundamental frequency ofoscillator 1002.

The filter 1004 is configured such that only the fundamental frequencyof the oscillator 1002 waveform passes through the filter, with most, orall, harmonics of the fundamental frequency being attenuated. Thus, ifthe output of the oscillator is a square waveform, the output of thefilter 1004 will be a sinusoidal waveform at the fundamental frequencyof the oscillator 1002. If the output of the oscillator 1002 is asinusoidal waveform, the output of the filter 1004 will also be asinusoidal waveform at the fundamental frequency of the oscillator 1002.The output of the filter 1004 is connected to a voltage controlledamplifier 1006.

The voltage controlled amplifier 1006 is configured to receive theessentially sinusoidal waveform output from the filter 1004. Inaddition, the voltage controlled amplifier 1006 is configured to receivea control signal from the feedback circuit 908. The control signal mayvary the gain of the voltage controlled amplifier 1006. In oneembodiment, the voltage controlled amplifier 1006 amplifies the waveformoutput by the filter 1004. In another embodiment, the voltage controlledamplifier 1006 attenuates the waveform output by the filter 1004.

In one embodiment, the output waveform of the voltage controlledamplifier 1006 is connected to a filter 1008. The filter 1008 attenuatesharmonics, or other undesired signals, that may have been generatedduring manipulation of the waveform in the voltage controlled amplifier1006. In one embodiment, the filter 1008 is a low pass filter with acorner frequency of about 7 MHz, a rolloff of approximately 12 dB peroctave. In another embodiment, the filter 1008 is a bandpass filtercentered at about the fundamental frequency of oscillator 1002,attenuated about 3 dB at approximately ±250 kHz from the fundamentalfrequency, and attenuated about 12 dB/octave It is desirable to selectfilter parameters so that there is steep attenuation of frequencies thatare out of band of the filter.

The output of the filter 1008 is connected to a safety switch 1010. Thesafety switch 1012 either passes the waveform received from the filter1008 on to a buffer amplifier 1014, or blocks the waveform, preventingit from reaching the buffer amplifier 1014 in response to a safetycontrol signal. In one embodiment, a safety control signal 1012 operatesthe safety switch 1010 in response to the remote switch 820. When theremote switch 820 is activated the safety switch will pass the waveformto the buffer amplifier 1014. When the remote switch 820 is inactivatedthe safety switch 1012 will block the waveform from buffer amplifier1014. In one embodiment, the safety switch 1010 is a relay. In otherembodiments the safety switch may be an active component, such as, forexample, bipolar or MOS transistors configured as gatable clamps. Inaddition, photo-resistors can be configured to perform the switchfunction as well as CMOS or MOS analog switch integrated circuits.

The output of safety switch 1010 is connected to the buffer amplifier1014. The buffer amplifier 1014 is configured as a unity gain amplifierused to improve the electrical load drive capability of the control unit902. In one embodiment, buffer amplifier 1014 is a high output drivecurrent buffer, such as, for example, National Semiconductor CLC5612.The output of the buffer amplifier 1014 is connected to the splitter910.

The feedback circuit 908 includes a signal conditioning and erroramplifier 1020, and a gating and duty factor control 1022. The signalconditioning and error amplifier 1020 produces a control signal thatcontrols the gain of the voltage controlled amplifier 1006. The signalconditioning and error amplifier 1020 control signal varies in responseto sensing and control inputs to the signal conditioning and erroramplifier 1020. In one embodiment, the voltage and current at theelectrosurgical tool 810 are sensed and transmitted to the signalconditioning and error amplifier. In addition, the user controls 904 mayallow a user to select desired voltage and current settings for anelectrosurgical procedure. For example, a user may select a maximumcurrent level that is not to be exceed, or a desired voltage level thatis desired to be maintained. Additionally, the user controls 904 mayallow a user to select desired gating and duty factor settings. Aspectsof voltage and current levels, as well as gating and duty factor onelectrosurgical procedures, is discussed further below.

As discussed above, for electrosurgical cutting to be effective anadequate, or critical voltage must be present at the cutting electrodeto sustain the vapor, or gas, barrier in a conductive state. If thevoltage present at the cutting electrode reduces to a level below thecritical voltage level, the vapor barrier will stop conducting, andcutting will cease. Regulation of the electrosurgical power generator tomaintain the voltage present at the cutting electrode at a level abovethe critical level is accomplished by monitoring the voltage present atthe electrosurgical tool 810 as well as the DC potential generatedacross the tool/tissue boundary and adjusting the gain of the voltagecontrolled amplifier 1006 accordingly.

In addition, regulation of the electrosurgical power generator outputvoltage can prevent the output voltage from increasing substantiallywhen tissue characteristics reduce the amount of current drawn from thegenerator. Limiting the voltage output of the electrosurgical powergenerator limits the energy transferred to the tissue and thereforereduces the risk of collateral damage to tissue during theelectrosurgical procedure.

The cutting current varies in response to tissue impedance changes.Lower impedance tissue, such as muscular or glandular tissue, has higherconductivity than higher impedance tissue such as fat, and thereforegenerally requires less sustaining voltage to produce the same amount ofcurrent. Factors that affect the impedance once cutting has beguninclude the electrode area and the conductance of a plasma layergenerated during the cutting process. A sustaining voltage, that variesas different tissue types are encountered, is needed to maintain thecurrent density over the electrode area.

Regulating, or limiting, output current from the electrosurgical powergenerator will reduce variations in the amount of current passed throughthe tissue as the electrosurgical tool 810 encounters tissue withdifferent impedance while cutting. Current regulation will reduce theamount of current passed through the tissue when lower impedance tissueis encountered and increase the amount of current, up to a presetcurrent limit, when higher impedance tissue is encountered.

The gating and duty factor control 1022 is in communication with theuser controls 904, and the signal conditioning and error amplifier 1020.In one embodiment, the gating and duty factor control 1022 modifies thewaveform used to generate the electrosurgical power generator output.Gating refers to allowing the signal from the low pass filter 1004 topass through the voltage controlled amplifier 1006. When the gate is“open”, or “on”, the signal passes through the voltage controlledamplifier 1006. When the gate is “closed”, or “off”, the signal does notpass through the voltage controlled amplifier 1006. Gating may be usedto allow a “burst” of signal through the voltage controlled amplifier1006. For example, the gating signal may turn on, and open the gate,allowing a desired number of cycles of the signal from the low passfilter 1004 through the voltage controlled amplifier 1008. The gate maythen turn off, closing the gate, and block further signals from the lowpass filter 1004 to pass through the voltage controlled amplifier 1006.In this manner, a burst of signals are allowed to pass through voltagecontrolled amplifier 1006.

Duty factor control refers to the ratio between the “on” and “off”periods of the gating signal. For example, a user may desire to have anoutput of the electrosurgical power generator 802 be a continuoussinusoidal wave of about 5 MHz that is modulated such that it appears asgated bursts at a controlled frequency when the remote switch isactivated. The user may desire the output of the electrosurgical powergenerator 802 to alternate between “on” and “off” at a 100 Hz rate withthe “on” period occupying 25% of the waveform period. To generate thiswaveform, the oscillator 1002 would generate a square wave or sine waveat 5 MHz. The oscillator 1002 output would pass through the low passfilter 1004. Output of the lowpass filter, a 5 MHz sine wave, will bepassed to the voltage controlled amplifier 1006. The gating and dutyfactor control 1022 generates a signal, passed to the signalconditioning and error amplifier 1020, so as to generate a controlsignal that will gate the voltage controlled amplifier 1006 on and offat a 100 Hz rate, thus repeating every 10 msec. The desired 25% dutyfactoring means that the voltage controlled amplifier 1006 will be gated“on” for 2.5 msec, and then gated off for 7.5 msec.

Gating may also be performed at higher frequencies, for example 50 kHz,or up to the frequency of the signal being gated. Gating at higherfrequencies may prevent subjecting the patient to frequencies within thebiological passband and thereby decrease the possibility ofneuromuscular stimulation.

Adjusting the waveform duty factor in this manner has several benefits,such as, for example, reducing the average power delivery to 25% of thepower that would be delivered with a continuous waveform. Although theaverage power is reduced by 25%, the peak voltage of the waveform duringthe “on” portion is unchanged. Thus, duty factoring may allow for asufficient voltage level to sustain cutting, while reducing the amountof energy delivered to the patient thereby reducing risks associatedwith excessive delivery of energy. Waveforms that can be used forcoagulation and blending may also be produced by controlling parametersas described above.

In one embodiment, when duty factoring is being used, the waveshapeenvelope is controlled to produce a desired waveshape. For example, awaveshape envelope may be, for example, a ramped, or trapezoidal,rectangular envelope. In another embodiment, the waveshape may be, forexample, a zero crossing switched rectangle. In other embodiments,different waveshape envelopes may be used to produce a desired signal.

FIG. 11 is a block diagram showing additional detail of portions of usercontrols 904 and gating and duty factor control 1022. In one embodiment,user controls 904 includes input switches. A first set of input switches1102 allows the user to select a desired gating, or repetition rate, forthe output of the electrosurgical power generator. The first set ofinput switches 1102 are buffered by a first set of logic buffer 1106 toisolate and enhance electrical drive capability to the input switches1102 signals. A second set of input switches 1104 allows the user toselect a desired duty factor for the output of the electrosurgical powergenerator. The second set of input switches 1104 are buffered by asecond set of logic buffer 1108 to isolate and enhance electricalcapability of the input switches 1104 signals.

The output of the first set of logic buffer 1106, the desired gating, orrepetition rate setting is communicated to the gating and duty factorcontrol 1022. The repetition rate setting is an 8 bit command that isconnected to the data inputs of two presettable counters, a lower nibblecounter and an upper nibble counter. The lower nibble counter and uppernibble counter are cascaded to produce a first 8 bit counter 1120. Inone embodiment, the presettable counters are 74HC163 integratedcircuits, or equivalent. The clock input to the counter 1120 isconnected to another presettable counter 1122 configured to divide themain clock to a desired frequency. In one embodiment the main clock is 5MHz and the counter 1122 is configured as a divide by four counter toproduce a 1.25 MHz output used to clock the counter 1120. In oneembodiment, the counter 1122 is a 74HC163 integrated circuit, orequivalent.

The ripple-carry output of the first 8 bit counter 1120 is connected toa D flip flop circuit 1126 that drives another set of presettablecounters cascaded to produce a second 8 bit counter 1130. In oneembodiment, the presettable counters are 74HC163 integrated circuits, orequivalent. The data outputs of the second 8 bit counter 1130 areconnected to a first set of 8 bit data inputs of an 8 bit comparator1140. The second set of data inputs of the 8 bit data comparator 1140are connected to the output of buffer amplifiers 1108 of user inputcontrols 904. The 8 bit data comparator 1140 produces a low logic leveloutput when the two eight bit data inputs are equal, and a high logiclevel output if the two eight bit data inputs are not equal. In oneembodiment, the data comparator 1140 is a 74HC688 integrated circuit, orequivalent.

The output of the 8 bit data comparator 1140 is communicated toadditional logic 1150 to provide electrical drive capability for theduty cycle command. Thus the output of logic 1150, the duty cycle, is ahigh logic output when the data outputs of the second 8 bit counter 1130equal the output from user input buffer amplifiers 1108, which representthe duty factor command. The output of logic 1150 will remain a highlogic level until the output of 8 bit counter 1130 changes as a resultof the ripple-carry output of the first 8 bit counter 1120 clocking thesecond 8 bit counter 1130. The ripple-carry output of the first 8 bitcounter 1120 will only clock the 8 bit counter 1130 after a selectednumber of clock cycles, representing a desired duration have occurred.In this manner the output of logic 1150 is a low logic level for theamount of time represented by switch 1104 settings, at a repetition rateas selected by switch 1102 settings.

FIG. 12 is a block diagram showing additional detail of portions ofsignal conditioning and error amplifier 1020. In one embodiment, signalconditioning and error amplifier 1020 includes a voltage sense amplifier1202 and a current sense amplifier 1204. Voltage sense amplifier 1202and current sense amplifier 1204 receive signals representing thevoltage and current, delivered by the electrosurgical power generator toan electrosurgical tool, respectively. The voltage sense amplifier 1202and the current sense amplifier 1204 are configured to produce a zero to5 VDC output representing zero to maximum voltage and currentrespectively.

Signal conditioning and error amplifier 1020 also includes a dutycontrol amplifier 1206. Duty control amplifier 1206 includes a digitalto analog converter configured to output a voltage representing adesired output of the electrosurgical power generator as commanded bythe user.

The output of the voltage sense amplifier 1202, the current senseamplifier 1204 and the duty control amplifier 1206 are combined at gaincontrol amplifier 1210. Thus, gain control amplifier 1210 produces again control signal used to control the voltage controlled amplifier1006 so as to produce the desired output from the electrosurgical powergenerator. The output of the generator is then controlled to provide adesired output level, with a maximum current limit established by aprogrammed current limit setpoint.

The gain control signal turns the voltage controller amplifier 1006 onand off in response to user inputs for gating and duty factor control.The voltage controlled amplifier 1006 is turned off when the gaincontrol signal is at zero voltage. In addition, when the voltagecontrolled amplifier 1006 is turned on, i.e. the gain control voltage isnon-zero, gain control signal will control the gain of voltagecontrolled amplifier 1002 in such a manner as to produce the desiredoutput of the electrosurgical power generator as reflected by thevoltage sense and current sense inputs to voltage sense amplifier 1202and current sense amplifier 1204 respectively.

FIG. 13 is a block diagram of an embodiment of a distal interface pod806. In one embodiment, the distal interface pod 806 includes a magneticcircuit 1302. The magnetic circuit 1302 is configured to receive RFpower from the electrosurgical power generator 802. The magnetic circuit1302 provides an impedance matching network, so as to provide a desiredload for the electrosurgical power generator 802. In one embodiment, themagnetic circuit 1302 provides a nominal impedance of 450 Ohms whendriven with a 50 Ohm source. In other embodiments different source andoutput impedances are possible. In addition, the transformer show inFIG. 13 is a non-isolating transformer. In another embodiment aconventional transformer with separate primary and secondary may beused.

The distal interface pod 806 may also include a current sense circuit1304 and a voltage sense circuit 1306. The current sense circuit 1304and the voltage sense circuit 1306 monitor the current and voltage thatare sent to the electrosurgical tool 810 and telemetry this data back tothe feedback circuit 908 in the electrosurgical power generator 802. Inone embodiment, the telemetry data are two voltage levels correspondingto current and voltage respectively. For example, the data correspondingto current may be a DC voltage scaled such that 0 to 1 VDC correspondsto 0 to 1 Amp. Data corresponding to voltage may be a DC voltage scaledsuch that 0 to 1 VDC corresponds to 0 to 120 Volts rms. In anotherembodiment the telemetry data are two current levels corresponding tocurrent and voltage respectively. For example, the data corresponding tocurrent may be a DC current scaled such that 0 to 20 mAmp corresponds to0 to 5 Amps. Data corresponding to voltage may be a DC current scaledsuch that 0 to 20 mAmp corresponds to 0 to 1000 Volts rms. In anotherembodiment, 0 to 5 volts corresponds to 0 to 1000 Volts rms and 0 to 5volts corresponds to 0 to 5 Amps.

The distal interface pod may also include a ground pad sense 1308 andground pad magnetic circuit 1310. The ground pad sense circuit 1308monitors the presence of the ground pad to ensure there is electricalconductivity between the patient 804 and the ground pad 814. The groundpad magnetic circuit 1310

FIG. 14 is an illustration of an embodiment of a flexible shielded cable812. The cable 812 is designed to carry up to 5 amps at up to about 14MHz. The cable 812 includes an inner conductor 1402. In one embodimentthe inner conductor 1402 is 26 AWG solid copper magnet wire. In otherembodiments, it is envisioned that other wire gauges, or stranded wirewill be used, such as, for example, 27 AWG, or 25 AWG. In anotherembodiment inner conductor 1402 may be plated with silver or gold due toskin effect of RF signals as they propagate in conductors. Completelysurrounding the inner conductor 1402 is an inner jacket 1404. In oneembodiment, the inner jacket 1404 is made of a flexible material such assilicone. In another embodiment, the inner jacket 1404 may be made fromHF material, such as, polypropylene. Between the inner jacket 1404 andinner conductor 1402 is a gap 1406. In one embodiment, the gap 1406 isfilled with air. In other embodiments, the gap 1406 may be filled withfoamed material, or solid material.

Surrounding the inner jacket 1404 is an electrical shield 1408. Theshield is typically grounded and reduces exposure of the personoperating the electrosurgical tool 810, as well as the patient 804, toRF radiation. In one embodiment, the shield is made from an electricallyconductive braid, or spiral wrap, providing a minimum coverage of 90%.In another embodiment the shield is made from an electrically conductivefoil. In other embodiments the shield may be a conductive fiberoverwrap, conductive coating of the center insulator or conductivecoating of the inner surface of the outer jacket with a drain wire.

Covering the shield 1408 is an outer jacket 1410. The outer jacket 1410protects the internal portions of the cable 812. In one embodiment, theouter jacket 1410 is made from silicone. In one embodiment, the cable812 is terminated at one end in a BNC connector, such as, for example,an AMP Economy Series P/N 414650 or equivalent.

The above described construction of the cable 812 minimizes exposure ofindividuals in the area near the cable to RF radiation while stillmaintaining flexibility to minimize interference with a surgeon'soperation of the electrosurgical tool 810. Flexibility is improved bythe ability of the cross section to distort in shape when bent. Theairgap reduces capacitance due to airs low dielectric constant.

There are two distinct phases in electrosurgical cutting: a startingphase; and a sustaining phase. When an inactive electrode of anelectrosurgical cutting tool is placed against tissue there isconductive coupling between the electrode and the tissue. Typically, theconductive coupling between the inactive electrode and the tissuepresents the lowest impedance load to the electrosurgical powergenerator during an electrosurgical procedure. When the electrosurgicalpower generator is activated it imposes a voltage, typically a voltagein the radio frequency (RF) portion of the electromagnetic spectrum, onthe electrode. The voltage imposed on the electrode causes current toflow through the tissue adjacent to the electrode. The current flowingthrough the tissue heats the tissue. The highest current density, andtherefore the most heating, is in the tissue closest to the electrode.

As the temperature of the tissue rises, the tissue begins to itdesiccate. During desiccation a steam layer will form between theelectrode and the tissue, increasing the impedance presented to theelectrosurgical power generator. If an adequate voltage is present onthe electrode, the steam layer will begin conducting current. Currentflows from the electrode through the steam layer and into the adjacenttissue. The current flow continues the process of desiccation of thetissue, and thereby continues the cutting of the tissue. With the onsetof the steam layer, the desiccation and cutting continue, and theelectrosurgical process enters the sustaining phase.

During the sustaining phase, cutting will continue as long as anadequate RF voltage is present at the electrode. Reducing the RF voltageon the electrode will end the sustaining phase, causing the cutting ofthe tissue to stop. Cessation of cutting may result in the deposition ofcarbon and biologic material on the electrode. This may result inre-establishment of proper cutting conditions more difficult. Increasingthe RF voltage above the level required to maintain the sustaining phaseresults in excessive power dissipation in the tissue and may lead to anincrease in collateral tissue damage.

The proper RF voltage for an electrosurgical procedure depends, in part,on the type of tissue encountered during the procedure. For example,experimentation has shown that typical muscular or glandular tissuepresents a nominal 200 to 300 ohm load to an electrosurgical powergenerator that is connected to a typical electrosurgical scalpel bladeor loop electrode. A typical electrosurgical scalpel may have a bladewith a cross section of 0.020 inches. During the surgical procedureapproximately 0.150 inches of the blade's length is typically in contactwith tissue. Thus approximately 0.003 square inches of theelectrosurgical blade are in contact with tissue. Test results indicatethat approximately 122 volts rms is required to begin, and sustain, acut in muscular or glandular tissue. The energy delivered into tissuewith an impedance of 300 ohms, at 122 volts rms, is approximately 50watts (P=E²/R=(122)²/300) and a corresponding energy density of 16,666watts/square inch (50 watts/0.003 square inches). In contrast tomuscular or glandular tissue, fat presents a nominal 450 to 800 ohm loadto an electrosurgical power generator connected to a typicalelectrosurgical scalpel or loop electrode as discussed above.

New electrosurgical tools may employ longer lengths of wire for cutting,such as those disclosed in the concurrently filed applications of thepresent assignee entitled “BIOPSY ANCHOR DEVICE WITH CUTTER” by Quicket. al., and “SHAPEABLE ELECTROSURGICAL SCALPEL” by Burbank et. al.,both filed Dec. 28, 2000 and both of which are incorporated herein intheir entirety.

For example, a new electrosurgical tool may employ a length of wire forcutting with a length of approximately 1.8 inches, and a cross sectionof 0.010 inches. Thus, 0.018 square inches of the cutting wire maycontact tissue when using the electrosurgical tool. In order to achievethe same energy density as produced by a conventional electrosurgicaltool, 16,666 watts/square inch, requires approximately 300 watts(16,666*0.018). To generate 300 watts into an 800 ohm load requires theelectrosurgical power generator to output nearly 490 volts rms(E=sqrt(PR)=sqrt(300*800)). Typical electrosurgical power generatorspresently available can only generate approximately 70 to 150 watts intoan 800 ohm load.

The waveform of the output of an electrosurgical power generator mayeffect the amount of energy transferred to the tissue, and thereby theefficiency of cutting, during an electrosurgical procedure. Conventionalelectrosurgical power generators typical output a waveform whichapproximates a squarewave, or a complex waveform. These waveforms aretypically harmonically rich and generally have a high crest factor, orratio of peak voltage to RMS voltage. In addition, conventionalelectrosurgical power generators generally produce output waveforms withfundamental frequencies in the range of 300 kHz to 1 MHz and powerlevels of 100 to 300 watts.

It has been observed during experimentation that cutting of tissue ismore effective using waveforms with higher frequencies, such as, forexample, about 1 MHz to about 14 MHz, particularly about 3 MHz to about8 MHz. Conventional electrosurgical power generators generally do nothave sufficient harmonic energy present, at these higher frequencies, toeffectively cut certain types of tissue, such as, for example, fattytissue. In addition, energy present at lower frequencies, that does noteffectively contribute to the cutting process, may be converted intoheat and lead to damage of collateral tissue.

Problems associated with cutting fat tissue are exacerbated if a largerelectrode is used, such as the electrodes disclosed in copendingapplications discussed above having about 0.010 to about 0.020 squareinches of contact area. In order to delivery enough high frequencyelectrical energy to the tissue, for cutting fat tissue, particularlywhen cutting with a large electrode, requires a electrosurgical powergenerator output waveform with sufficient high frequency energy. Inaddition, to reduce the risk of potential damage to collateral tissue,lower frequency energy that does not effectively cut tissue needs to beminimized.

In one electrical power generator embodying features of the invention,the output waveform is essentially a sinusoidal waveform. As usedherein, reference to an essentially sinusoidal waveform is a waveformwith less than about 5% total harmonic distortion (THD). A sinusoidalwaveform at a high frequency, for example between about 1 MHz and about14 MHz, and at a power level up to 1,000 watts has an advantage ofdelivering electrical energy at a frequency most effective for cuttingacross a wide variety of tissue types, while minimizing the amount ofenergy delivered that is not effective in cutting, but rather leads todamage of collateral tissue.

The foregoing description details certain embodiments of the inventionso that an understanding of the present invention can be conveyed. Itwill be appreciated, however, that no matter how detailed the foregoingappears, the invention may be embodied in other specific forms withoutdeparting from its spirit or essential characteristics. Moreover, thoseskilled in the art will recognize that features shown in one embodimentmay be utilized in other embodiments. The described embodiments are tobe considered in all respects only as illustrative and not restrictiveand the scope of the invention is, therefore, indicated by the appendedclaims rather than by the foregoing description.

What is claimed is:
 1. A method for controlling electrosurgical tissuecutting at a patient's site, comprising: a. providing an electrosurgicalcutting tool having a tissue cutting electrode with a distal tip and anexposed conductive length proximal to the distal tip that is configuredto contact tissue; b. providing a return electrode in contact with thepatient's tissue remote from the patient's site; c. providing anelectrosurgical RF power generator in an electrical conductiverelationship with the tissue cutting electrode and the return electrode;d. contacting tissue with the exposed conductive length of the tissuecutting electrode; e. generating RF energy as a waveform within theelectrosurgical RF power generator and gating the RF energy to generategated RF energy having a duty cycle of less than 100%; f. delivering thegated RF energy from the electrosurgical RF power generator to thetissue cutting electrode so as to pass an electrical current from theexposed conductive length of the tissue cutting electrode to tissue incontact with the exposed conductive length of the tissue cuttingelectrode to form a steam layer between the tissue cutting electrode andthe tissue by tissue desiccation and generate a conductive plasma alongthe exposed conductive length, and with the onset of the steam layercontinued desiccation and cutting of the tissue is effected; and g.regulating the electrosurgical RF power generator to maintain a voltagepresent at the tissue cutting electrode above a level needed to maintainformation of the steam layer by adjusting a gain of a voltage controlledamplifier in the electrosurgical RF power generator based at least inpart on a voltage present at the electrosurgical cutting tool as well asa DC potential generated across a tool/tissue boundary of theelectrosurgical cutting tool and the tissue.
 2. The method of claim 1wherein the electrosurgical RF power generator has a start mode when thevoltage at the tissue cutting electrode is less than the level to cuttissue.
 3. The method of claim 2 wherein the electrosurgical RF powergenerator has a sustaining mode when the voltage is at or exceeds thelevel to cut tissue.
 4. The method of claim 1 wherein the exposedconductive length of the tissue cutting electrode has an arcuate shape.5. The method of claim 4 wherein the arcuate shaped exposed conductivelength of the tissue cutting electrode is moved to cut tissue byrotation.
 6. A method for controlling an electrosurgical RF powergenerator for performing electrosurgical tissue cutting at a patient'ssite, comprising: a. directly contacting tissue at the patient's sitewith an exposed conductive length of a tissue cutting electrode while areturn electrode is in contact with the patient's tissue at a locationthat is remote from the patient's site, each of the tissue cuttingelectrode and the return electrode being in electrical communicationwith the electrosurgical RF power generator; b. generating RF energy asa waveform within the electrosurgical RF power generator and gating theRF energy to generate gated RF energy; c. delivering the gated RF energyfrom the electrosurgical RF power generator to the tissue cuttingelectrode so as to pass an electrical current from the exposedconductive length of the tissue cutting electrode to tissue in contactwith the exposed conductive length of the tissue cutting electrode toform a steam layer between the tissue cutting electrode and the tissueby tissue desiccation and generate a conductive plasma along the exposedconductive length, and with the onset of the steam layer continueddesiccation and cutting of the tissue is effected; and d. regulating theelectrosurgical RF power generator to maintain formation of the steamlayer by adjusting a gain of a voltage controlled amplifier in theelectrosurgical RF power generator based at least in part on a DCpotential generated across a tool/tissue boundary of the electrosurgicalcutting tool and the tissue at the patient's site.
 7. A method forcontrolling an electrosurgical RF power generator for performingelectrosurgical tissue cutting at a surgical site on a patient,comprising: a. contacting tissue at the surgical site with an exposedconductive length of a tissue cutting electrode while a return electrodeis in contact with the patient's tissue at a location that is remotefrom the surgical site, each of the tissue cutting electrode and thereturn electrode being in electrical communication with theelectrosurgical RF power generator; b. generating RF energy as awaveform within the electrosurgical RF power generator and gating the RFenergy to generate gated RF energy; c. delivering the gated RF energyfrom the electrosurgical RF power generator to the tissue cuttingelectrode so as to pass an electrical current from the exposedconductive length of the tissue cutting electrode to tissue in contactwith the exposed conductive length of the tissue cutting electrode toform a steam layer between the tissue cutting electrode and the tissueby tissue desiccation and generate a conductive plasma along the exposedconductive length, and with the onset of the steam layer continueddesiccation and cutting of the tissue is effected; and d. regulating theelectrosurgical RF power generator to maintain formation of the steamlayer by adjusting a gain of a voltage controlled amplifier in theelectrosurgical RF power generator based at least in part on a DCpotential generated across a tool/tissue boundary of the electrosurgicalcutting tool and the tissue at the patient's site.